Un Helico (oz16580)

About this Plan

Un Helico. Free flight sport model helicopter for rubber power.

Quote: "Un Helico. Fascinating full size plans for a Peanut Helicopter by French master-modeller Emmanuel Fillon.

THIS INTERESTING free flight helicopter, by well known French designer Emmanuel Fillon, seemed too good to miss... The drawings are shown full size for you - the more experimental modeller, to start building right away - or as food for thought, should you wish to choose your own full-size subject to miniaturise.

The model: The Peanut Scale model appears to be based on the Bell-Huey 'Cobra' - a slim machine, well suited to modelling (our radio control compatriots used the 'Cobra' in the days when R/C 'chopper' flying began). It uses simple rubber power, but unlike the traditional direct drive 'stick and rotor' duration models, has a gearbox to divide the power between main and tail rotor in the correct ratio.

Drive system: Power is transmitted from the gearbox via spring drives and shafts. Each spring has a loose curved guide wire inside, to prevent the springs kinking under stress. Do not, however, expect to wind the rubber via the rotors, this might strain the transmission. A locking pin has to be inserted via plastic tubes in the fuselage through a loop in the drive shaft.

This holds the rotors still and provides a safe hold, whilst winding is done from the noseblock end. Once wound, the notch in the nose block engages with the fuselage and restrains this against rotation, so that all is locked up until the moment of launch, when the locking pin is withdrawn.

Rotor head: The gimbal mounted rotor can rock freely, so that the tips may rise and fall and the blades (which are rigidly fixed together) can change their angle of attack relative to the flight path.

Without going deeply into the theory of helicopter flight, it is essential that the rotor disc (path of the blades) is tilted in the direction of horizontal flight (forward for forward flight, or to combat a breeze) and, in this case, to one side to promote circling flight (when combined with tail rotor effort).

What stops the blades flapping about? There are a pair of small weights on a rod called a "flybar" set at 90° to the blades (in plan view). Their job is to maintain the angle of attack relative to the rotor disc.

In a full-size machine or R/C model, control is exercised by tilting the path of these weights (which controls the path of the blades) by means of a swash plate. This model relies on a slight tilt, for cruising, rather than pure vertical climb. Any R/C flier will tell you that it is easier to control a model like this - Hovering is more difficult. The best trim for this model is a slow circle.

Torque: The tail rotor has adjustable pitch, which must be set by trial and error to combat torque. It pushes the tail to the right against torque - coarse pitch should cause LEVI' yaw; fine pitch, RIGHT yaw. The main rotor shaft is inclined to the left, to give a left circle, so use slightly coarse pitch. If Right yaw is combined with Left rotor 'tilt', then the model would be difficult to trim.

Ballast and balance: Initially the model should be balanced exactly on the rotor shaft. Small pieces of plasticene can be added to either or both landing skids to tilt the whole model in the direction required for cruise.

It is essential that the main rotor is perfectly balanced in all planes - spin it freely in the horizontal and vertical axis to test. The coning angle of the blades (dihedral to you fixed wing types) must bring their tips up to balance the root ends which are below their pivot, each blade must be the same weight (add dope or plastic tape to the lighter one) and the flybar weights must be the same.

When all is correctly set up, there should be little or no vibration - vibration makes trimming of even R/C choppers difficult or impossible, so give yourself a chance with this one.

Construction tips: That fuselage has to be feather light - it needs patience when hollowing out after shaping the outside. Hold it up to the light to judge the remaining thickness. If a hot wire cutter is used; err on the cooler side, otherwise a thick skin of melted expanded polystyrene will negate the weight saving -the material is not actually removed merely re-formed in a more solid form. Readers may like to use a 1/32 or 1/64 balsa box fuselage of more angular form, but try to keep the whole model down to under 25grams (less rubber)."

Direct submission to Outerzone.

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